United Kingdom uses similar GSM mobile networks (GSM 1800 and GSM 900) as France so you should be able to normally use your mobile phone.

Roaming

According to the EU regulations your messages, calls and Internet mobile data in United Kingdom will cost the same as in France. Note that there is fair-use policy which means that the use is limited to business & leisure.

Data

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Common data package size in GB (Gigabyte) and it's price, common carriers and network capabilities.